T 7.2.1.3 Amplitude Modulation

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TPS 7.2.1.3 Single Sideband AM The overlapping transmission range has to run symmetrically with respect to the carrier frequency. Demodulation method : Synchronous demodulation Bandwidth requirement : f Mmax < b < 2 f Mmax Application : TV technology s f C + f M f C f C + f M (f) Experiment procedure Set up the experiment as specified in Fig. 6-4. Connect the output of the function generator directly to the AF input of the modulator M2. Set the function generator to: sinusoidal, A M = 2 V and f M = 2 kHz. 6.1 Investigations on the dynamic characteristic of the SSB 6.1.1 SSB RC Set the toggle switch to CARRIER ON. Display the output signal of the channel filter CH2 and the modulating signal s M (t) of the function generator on the oscilloscope and sketch the signals. (Modulation product on channel 2, modulating signal on channel 1 of the oscilloscope). Use Diagram 6.1.1- 1. Shift the AF signal to the upper or lower envelope curve of the AM signal. Vary the frequency f M and the amplitude A M of the modulating signal. What do you observe? Settings on the oscilloscope Input attenuator channel 1 Input attenuator channel 2 Time base Trigger 2 V/DIV 1 V/DIV 200 µs/DIV mod. signal Fig. 6-3: Filter characteristic with Nyquist slope for VSB Trigger to the modulating signal s M (t). Switch the modulating signal off. Measure the unattenuated carrier amplitude A C at the input of the channel filter, as well as the amplitude A RC of the attenuated carrier at the output of the channel filter. Calculate the ratio k = A RC /A C . Determine the carrier suppression t in dB according to (6-4): m t = 20 log (6-4) 2 k 6.1.2 SSB SC Set the toggle switch to CARRIER OFF. Set the oscilloscope as specified in the Table. Proceed as described in point 6.1.1. Use Diagram 6.1.2-1. What features does the SSB SC signal have? Settings on the oscilloscope Input attenuator channel 1 Input attenuator channel 2 Time base Trigger / external 2 V/DIV 1 V/DIV 200 µs/DIV mod. signal Diagram 6.1.1-1: Dynamic characteristic of the SSB RC Signals (1): Modulating signal s M (t) (2): Modulation product at the output of CH2 Diagram 6.1.2-1: Dynamic characteristic of the SSB SC (1): Modulating signal s M (t) (2): Modulation product at the output of CH2 42
% 0 TPS 7.2.1.3 Single Sideband AM 6.2 Spectrum of the SSB 6.2.1 SSB RC Set the toggle switch to CARRIER ON. GATE 1s 10s 0,1s 0,01s TIME A-B COUNT A CHECK FUNCTION RATIO A/B PERIOD A FREQ A TTL - IN(A) TTL - IN(A) ANALOG (A) TTL - IN(B) Analyzer settings V 1 :1 V 2 :10 f r /kHz: 50 b/Hz: 100 SPAN/kHz:1.5 ... 20 T/s:40 Set the spectrum analyzer as specified in the Table. Connect its input to the output of the modulator M2. As the modulating signal use a sinusoidal signal with A M = 2 V and f M = 2 kHz. Feed the modulating signal into the input filter of the CF transmitter. Measure the SSB spectrum in the range of approx. 15 kHz up to 25 kHz and enter the measured values S(n) from the output of the analyzer with their corresponding frequencies in Table 6.2.1-1. The amplitude values of the wanted spectral components are obtained from: S AM S(n) (n) = V ⋅ V 1 2 Calculate the spectral components S AM (n) with the aid of (3-8). Also enter the calculated values for S AM (n) in Table 6.2.1-1. Plot the curve of the spectrum in Diagram 6.2.1-1. Label the spectral lines. Determine the transmission bandwidth of the AM signal based on the measurements. Generalize the results for the case of any modulating signals. kHz V pp = DC MODE FUNCTION OUT ATT dB 20 40 TTL 6.2.2 SSB SC Set the toggle switch to CARRIER OFF. Use a sinusoidal signal with A M = 2 V and f M = 2 kHz as a modulating signal. Measure the spectrum as described in point 6.2.1. Enter your results in Table 6.2.2-1 and Diagram 6.2.2-1. Evaluate your measurement results as shown in point 6.2.1. I > I > U U +15V (+5V) 0V M1 Fig. 6-4: Experiment setup for SSB 43
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T 7.2.1.3 Amplitude Modulation

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